Can't answer your question, but I've always been told that metabisulphite isn't a good sanitizer for beer. It works for wine because of the lower pH and higher alcohol content. Is that incorrect?
Denny, you're right if you're talking about knocking back wild yeasts in wort vs must. But ynot's asking about making a sanitizer with a high concentration of sulfite in water, which is common (in winemaking anyway).
I heard two minutes is fine. Add some to your carboy and shake, then let it sit with a stopper on. The sulfur fumes do as much of the work as the liquid. Many also add citric acid to the water bring the pH down - a low pH makes the sulfite work better.
I'm not sure why your LHBS recommended sulfites over other sanitizers. I don't think it's necessarily better than star-san or other sanitizers. I think you can get away with one set if you are super careful about cleaning. You could get a stainless steel racking cane and just keep a second set of tubes.